287
FXUS63 KGRB 180343
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1043 PM CDT Fri May 17 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Dense fog is possible over the lakeshore counties and the
  nearshore waters of Lake Michigan late tonight through Saturday
  morning.

- There is a chance of thunderstorms in north-central Wisconsin
  through early this evening.

- There is a chance of thunderstorms Saturday afternoon and
  evening. Some of these storms may become strong to severe across
  north-central and central Wisconsin.

- Additional shower and thunderstorm chances are expected early
  next week. While too early to determine any severe risk, there
  is an increasing likelihood for locally heavy rain to occur.

- Warm temperatures expected this weekend into early next week.

&&

.DISCUSSION...
Issued at 215 PM CDT Fri May 17 2024

Short Term...Tonight and Saturday

An upper trough will track through the northern Great Lakes
through this evening, which could produce an isolated shower or
thunderstorm across far north-central Wisconsin through early this
evening. Upstream radar reflectivity is rather unimpressive with
high based clouds associated with this feature as it is moving
through northwest Wisconsin. Model soundings indicate rather
meager instability as the trough moves through north-central
Wisconsin, with MUCAPEs on the order of 300-600 J/kg, and bulk
shear around 30 knots. In addition, there is a very dry airmass
in place across that area, as evidenced by dew points of 45 to 50
degrees. Therefore, confidence is low (20-30 percent) in anything
developing across far north-central Wisconsin as this feature
moves through and if storms do develop, they are not expected to
be severe.

Mostly clear skies and light winds should prevail across the
region overnight as southeasterly flow keeps overnight lows on the
warm side. The southeasterly flow could once again bring in some
fog from Lake Michigan later tonight into early Saturday morning,
with models indicating the impacts will mainly be confined to the
lakeshore counties. Given tonight will be another day removed from
the precipitation from the day and night before, this seems like a
reasonable solution. Lows tonight are expected to mainly be in the
lower to middle 50s inland, with upper 40s near the lakeshore.

Saturday is expected to be a warm day across northeast Wisconsin
as surface winds turn south or south-southwest and 850 mb
temperatures rise to 13-16 degrees Celsius. This will allow
daytime highs to climb to around 80 across north-central, with low
to mid 80s across central and east-central Wisconsin, and cooler
temperatures in the 70s or 60s near the lakeshore. Despite the
warm temperatures, dew points in the 50s should limit the humidity
across the region on Saturday and keep heat index values a few
degrees cooler than the ambient temperature.

An approaching cold front will bring the chance for showers and
thunderstorms to central and north-central Wisconsin from west to
east late Saturday afternoon. A ribbon of decent instability,
with MUCAPEs around 1000 J/kg, and bulk shear values of 20 to 30
knots are expected ahead of the approaching front. Given the
better instability Saturday afternoon, along with a surface cold
front, there is a chance (5% probability) that these storms could
become severe with damaging winds and large hail the primary
threat west of the Fox Valley.

Long Term...Saturday Night Through Friday

Models look consistent in showing an eastern CONUS upper ridge/
western CONUS upper trough developing this weekend and persisting
through Tuesday. This pattern will bring a southwest flow aloft
into WI, bringing a warm, moist air mass into WI. Several mid-
level shortwave troughs are forecast to eject from the trough and
potentially bring several rounds of locally heavy rain to
northeast WI. Eventually, this trough gets kicked out of the
western CONUS into the Great Lakes region mid-week with more rain
chances. Temperatures to be above normal through Tuesday, then
settle below normal during the latter part of next week.

Saturday night and Sunday...
Scattered showers with diminishing thunderstorms to accompany the
cold front/mid-level shortwave trough across the forecast area
Saturday evening before shifting to the east overnight. Drier air
advecting into the region behind the cold front will bring
decreasing clouds later Saturday night and allow for min
temperatures to range from the middle to upper 40s north-central
WI, to the middle to upper 50s east-central WI. A weak area of
high pressure to drift across WI on Sunday, providing for a sunny
start to the day. However, clouds are expected to gradually
increase in the afternoon as moisture begins to gather in the
vicinity of the now-stalled cold front situated over the Midwest.
Temperatures will remain on the warm side away from Lake MI with
readings in the middle 70s to around 80 degrees (upper 60s to
lower 70s lakeside due to onshore winds).

Sunday night and Monday...
The stalled boundary becomes a warm front and starts to lift
northeast toward western sections of the Great Lakes Sunday night.
Meanwhile, the first of several mid-level shortwave troughs will
lift into the Midwest within the southwest flow aloft. Anticipate
increasing isentropic lift and moisture transport to bring rain
chances back to northeast WI after midnight, especially over
central WI. Min temperatures to be in the lower 50s north/near
Lake MI, middle to upper 50s south. This shortwave trough is
forecast to lift northeast through the western Great Lakes and
when combined with persistent isentropic lift, anticipate showers
to be likely across all the forecast area on Monday. There will be
a chance for thunderstorms as well, but both instability and shear
look weak, thus severe storms look unlikely. Max temperatures
Monday to be in the upper 60s to around 70 degrees near Lake MI,
lower to middle 70s north and middle 70s to around 80 degrees
south.

Monday night and Tuesday...
Low-end precipitation chances look possible Monday night as models
indicate a frontal boundary to linger over WI. Showers and
thunderstorms to increase again on Tuesday as the next shortwave
trough moves northeast from the central Plains and will be
accompanied by a strengthening area of low pressure and cold
front. The gulf will be wide open and a 50 kt south-southwest
low-level jet should easily bring moisture northward into the
Great Lakes. PW values reach around 1.5 inches, so locally heavy
rain is possible. Severe storm potential is conditional depending
on instability. Bulk shear values of 40+ kts may allow for some
storms to become strong, so will need to watch this system in the
coming days. Max temperatures Tuesday to range from the middle to
upper 60s lakeside, mainly lower to middle 70s inland.

Tuesday night and Wednesday...
Showers/thunderstorms likely to persist through at least Tuesday
evening depending on which model(s) ends up correct as this system
continues to track northeast toward southern Ontario and the cold
front sweeps east. By Wednesday, the mean flow begins to shift as
the western upper trough quickly moves east and is already
approaching the western Great Lakes by Wednesday afternoon. There
does not appear to be any frontal boundaries in the vicinity and
gulf moisture to have been shunted east. Precipitation trends look
down for Wednesday, but with that upper trough not far away,
cannot completely dismiss precipitation chances for now. Max
temperatures for Wednesday to cool a bit with lower to middle 60s
north-central, middle 60s to near 70 degrees elsewhere.

Wednesday night and Thursday...
Weak high pressure will attempt to bring a break to precipitation
chances Wednesday night into Thursday. Mean flow to be nearly
zonal by this time, thus max temperatures on Thursday to be close
to normal with lower to middle 60s north-central/near Lake MI,
middle to upper 60s elsewhere.

Thursday night and Friday...
Serious model timing issues arise late next week with the next
shortwave trough/cold front. A slower movement would keep Friday
dry, while a faster movement would bring rain chances back as
early as late Thursday night. Have followed the blended model
solution and will bring small chance pops into the area on Friday.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1043 PM CDT Fri May 17 2024

VFR conditions prevailed during the evening hours. Mainly VFR
conditions are expected tonight into Saturday morning with only
some mid clouds expected at times. Exception will be near Lake
Michigan (possibly impacting MTW/2P2/3D2/SUE) where dense fog and
low clouds are expected overnight into Saturday morning. Areas of
dense fog have persisted across parts of southern Lake Michigan
this evening, and this area is expected to shift/expand northward
overnight. Some uncertainty to just how fast this will occur and
if it will push inland at all. No evidence of the fog expanding
north as of yet and models have backed off on the inland extent,
so have backed off the arrival of the lower visibilities and
removed the low ceilings. Low chance (under 30%) that some patchy
ground fog may form further inland. Fog trends will need to be
monitored overnight.

A few high based showers or sprinkles are possible across central
and north-central WI overnight into early Saturday but should
have no impact on ceilings/visibilities. A broken line of
showers and isolated/scattered storms is expected to push west to
east across the area Saturday afternoon and early evening,
arriving at AUW/CWA/RHI in the 22z to 00z timeframe and
GRB/ATW/MTW between 01-03z. Will include thunder in the TEMPO
groups for central and north-central WI, but hold off further east
as instability will be lower/decreasing as the activity pushes
east and/or the storms may stay just to the north of these sites.

Light winds are expected the rest of the night. Low-end LLWS will
be possible at RHI overnight as winds at 2000 ft increase to
30-35 kts. South winds will increase Saturday morning, gusting to
25 kts by late morning, with some higher gusts possible in the
afternoon.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....Kurimski/Kallas
AVIATION.......Bersch